Osseointegrated dental implants, an alternative to a single tooth, fixed bridge or removable dentures, have been developed as the result of the discovery by Dr. Branemark in Sweden of the ability of bone to make direct biological attachment to the surfaces of titanium implants. (See "Dental Implants: Tissue-Integrated Prosthesis Utilizing the Osseointegration Concept" published in the Mayo Clinic Proceedings, vol. 61 Feb. 1986, pp 91-97).
The procedure involves two operations.
The first operation involves surgically exposing the bone and then drilling the surface of bone where the prosthetic tooth is to be attached and inserting an implant (a titanium anchor) having a threaded or nonthreaded plug into the hole in the bone. A flap of skin is then sewn over the site. During the healing period that follows, the bone becomes biologically attached to the surface of the implant.
In the second operation, after the implant has become thoroughly attached to the bone, the flap is removed. An abutment, which is simply an anchoring stud that serves as a spacer, is screwed onto the implant for ultimate attachment to the prosthetic tooth.
In order to achieve the desired arrangement of prosthetic teeth in the mouth, a "negative" impression is made of the dental process which includes the implants, remaining teeth and gums. In this procedure, a "coping" (temporary stud) is attached at the site of each individual implant in place of the threaded plug. An impression material contained in an impression tray is imposed around the coping and dental process. The impression material hardens and then the tray with impression material is removed. U.S. Pat. No. 4,693,683 to Lee and U.S. Pat. No. 4,432,728 to Sharkey describe dental trays and techniques for making impressions of dental processes.
Either one of two types of copings is used, a "Square coping" or a "Round coping". These copings are described as follows:
The square coping is used with a custom made impression tray that has an opening that allows access to a placement pin that attaches the square coping through a temporary or "analog" abutment to the implant. After the impression material has set, the placement pin is removed thus permitting removal of the custom tray from the patient's mouth with the square coping imbedded in the impression material. A positive stone cast model of the patient's dental process is made in which an "analog" abutment (a substitute for the final abutment in the patient's mouth,) is temporarily attached to the square coping while the master model is being formed around the negative impression.
A disadvantage with the square coping is that it will move within the impression material when placing a guide pin and analog abutment together despite efforts such as intertwining dental floss between copings.
The round coping is placed directly on the implant. A tray with impression material is then placed over the round coping and after the impression material has hardened, the tray and material are removed leaving the round coping still on the implant. The round coping must be reinserted into the molded impression in order to make the master model
A disadvantage with the round coping is that there is a technique difficulty in returning the coping fixture into the impression material in a precise fashion.
The problem with the use of square and round copings is exaccerbated by distortion of the impression material as the material sets. The distortion is directly proportional to the mass of the material mass, i.e., the greater the mass, the greater the distortion.
The result of these problems is that the use of copings to form dental implant impressions, although long practiced, rarely produces an accurate impression.